The invention relates to an arrangement for examining a body with ionizing radiation, comprising means for scanning a spacial region into which the body region to be examined can be brought, from numerous angular positions with a beam having a pencil-like to a fan-like formation, with radiation detectors arranged on the opposite side of the spacial region in a centered or aligned fashion so as to receive the transmitted beam. As is well known in the art, a computer may be provided for processing the output signals from the detectors, so that the output signals may be stored in the form of digitally coded numerical information from which a matrix of measured attenuation coefficients may be obtained.
In recent years, arrangements of this type have become known as so-called computer tomographs. The radiation source used is an x-ray tube which is operated at a constant dose rate. Usually the radiation is collimated into a thin pencil-shaped beam. This beam is scanned over the body region to be examined along the layer plane by means of parallel movement or displacement of the x-ray tube together with the radiation detector. The radiation detector measures the dose rate of the x-ray beam which is transmitted through the body under examination. Since the dose rate of the non-attenuated beam is known, the attenuation of the dose rate can be determined from the two values. When the penetration density is known, the attenuation coefficient of the particular segmental region penetrated by the rays can be calculated. The spacial region is penetrated by the rays from a large plurality of directions. From the measured values obtained in this way, it is possible to calculate and print out or record a distribution of the attenuation coefficients for the respective spacial elements, that is, a matrix of attenuation coefficients, by way of an approximate calculation carried out in a computer. A matrix of attenuation coefficients of this type, that is a computer tomogram, may be displayed, for example, in gray tones or color gradations on the fluorescent screen of a television display unit. As so displayed, the computer tomogram shows not only the external contours of the body to be examined, but also the contours of the tissues or organs contained therein in gradations of their absorption coefficients. An image display such as this can be diagnostically evaluated in the same way as a transverse layer photograph.
In radiotherapy, a computer tomogram of this type is just as helpful for drawing up an irradiation plan as a conventional transverse layer photograph. However, the information content of a computer tomogram of this type is not sufficient for working out an irradiation plan with a specific selected isodose distribution.
1. The measured attenuation coefficients apply solely to the energy of the x-radiation which is used to generate the computer tomograph. However, much harder x-radiation or even gamma radiation from radioisotopes is used in radiation therapy. Therefore, the measured attenuation coefficients are not valid for this hard, penetrating radiation. Nor can they be readily converted, because the attenuation coefficient is dependent in a complex manner upon the density of the tissue, the energy of the x-radiation, and the atomic number of the elements contained in the individual tissue section, and at least two of these variables are unknown.
2. The attenuation coefficients measured in the computer tomograph are already invalidated in the region of the body to be examined as a consequence of the inevitable hardening of the ray spectrum as the rays penetrate the body. This invalidation of the attenuation coefficients is indeed usually corrected by an estimated correction factor which can be fed into the computer. The inaccuracies in the absolute attenuation coefficients connected with the estimation scarcely interfere in diagnostic evaluation. However, they would be obstructive in the drawing up of an irradiation plan. For all these reasons, it has repeatedly been necessary in the past to resort to elaborate experiments in order to determine a reasonably coordinated irradiation plan. The work involved increases exponentially with the improvement in the quality of the irradiation plan which is to be attained.